The proposed research focuses on the analysis of molecules and molecular assemblies involved in control and recognition both at the surface and within cells of immune system. In a coordinated effort, research in four major areas will be carried out: 1) to examine various hypotheses on surface modulation and to characterize the molecules involved in transmembrane control; 2) to describe, using biochemical techniques, the pathway for mitogenic stimulation, to link events at the cell surface with the initiation of DNA synthesis; 3) to study the role of lymphocyte surface molecules involved in recognition events by examining the function of H-2 antigens in T cell-mediated cytotoxicity and by defining the genetic controls that regulate the expression of surface molecules during ontogeny of the immune system; 4) to describe important interactions at the lymphocyte surface by using X-ray crystallographic and electron microscopic techniques to examine carbohydrate-binding ligands and Beta2-microglobulin. Results of such studies should provide some insight into the nature of recognition and control events in lymphocyte function and thus aid in answering questions in cellular immunology involving the molecular basis of clonal selection, the role of H-2 antigens in cellular recognition and development, and biochemical and regulatory processes in lymphocyte division.